Acoustic logging is frequently used in oil and gas operations to determine various properties of an earth formation in which a borehole has been drilled. Many acoustic logging data processing and analysis techniques were developed in conjunction with wireline acoustic logging tools, which are run in the wellbore after drilling is completed. These tools are operatively electrically connected to surface processing equipment by the wireline, which allows relatively large quantities of acoustic data to be transmitted to the surface for analysis. With the advent of measuring while drilling (MWD) and/or logging while drilling (LWD) systems, the wireline connection was no longer available. (Throughout this document LWD will be used to refer to both MWD and LWD systems.) Although there are a variety of techniques for communicating with LWD tools during the drilling operation, including, for example, electromagnetic and mud pulse telemetry, these channels tend to be somewhat bandwidth constrained as compared to wireline applications. As a result, many of the data processing and analysis techniques that were developed using wireline tools were adapted to perform more processing downhole and limit the amount of data that is transmitted to the surface.
For example, acoustic logging is often undertaken to determine compressional and shear wave velocities of the formation. These velocities can subsequently be used to determine other parameters of interest, such as, porosity, lithology, and pore pressure, all of which relate to the amount of oil or other hydrocarbons in the formation and/or the ease with which the hydrocarbons can be recovered. The velocities (as well as Stonely velocities and other parameters) can be determined as a function of depth using a technique known as semblance processing. Advances in downhole tool design and capabilities have permitted better semblance processing results to be generated downhole, yet the problem of getting this data to the surface remains. Historically, various (usually lossy) compression techniques have been used. Unfortunately, these techniques have often resulted in less-than-optimal results, as too much data is sacrificed to comply with bandwidth limits. The data lost as a result of these techniques can often lead to ambiguities in the data transmitted to drilling engineers at the surface, resulting in sub-optimal decisions relating to both the steering of the wellbore and appropriate completions techniques. Thus, what is needed is a better technique for compressing acoustic data measured and/or generated by a downhole LWD system so that more and/or better information can be transmitted to the surface despite the constraints of commonly used downhole telemetry systems. Although disclosed in the context of LWD systems, such data compression techniques could also be used in wireline systems.